It is known that the optimum valve lift and lift timing for intake and exhaust in an engine differ according to engine running conditions.
To provide an optimum valve lift, Tokkai Sho 62-294709, published by the Japanese Patent Office, discloses a variable cam engine wherein several cams are selectively applied to one valve.
In this engine, a cam which gives high power on low engine speeds and a cam which gives high power on high engine speeds are selected alternately. The change-over between these cams is performed by oil hydraulic pressure, and for this purpose, part of the lubricating oil discharged from an oil pump attached to the engine is led into the cam change-over mechanism via a valve.
In order to perform cam change-over using lubricating oil in this way, a fairly large amount of oil has to be discharged from the pump. However, most oil pumps do not supply the requisite amount of oil until the engine speed has increased to approx. 3000 rpm, and when the engine speed is lower than this value, cam change-over either cannot take place or takes too long.